Method for the manufacture of a rocket motor

ABSTRACT

Method for the manufacture of a rocket motor having a glassfilament wound casing involving the steps of forming the motor in at least two sections and providing for telescopic engagement of the casing of each section, providing abutting surface for the propellant charge, inhibiting the abutting surfaces, telescopically engaging the casing of each section and bonding the inhibited and abutting surfaces of the propellant charge, and peripherally bonding the casing to form a unitary motor.

United States Patent [72] Inventors John A. Leasure. Jr.; [56] References Cited John H. Main. Salt Lake City, Utah; UNITED STATES PATENTS H A I No 22%;; 3,017,743 1/1962 Adelmon 86/1X 1 3,088,273 5/1963 Adelman et a1 60/35.6 [22] Filed May 5, 1965 1122,884 3/1964 Grover etal... 60/35.6 DlVlSlOll ofSer. No. 104,469, Apr. 20, 1961 3 174 388 3/1965 Gaubatz 86/1 Patented Jan 19,1971 [73] Assignee Hercules In or orat d Primary Examiner-Benjamin A. Borchelt a cor oration of Delawa Assistant Examiner-Stephen Cv Bentley Attorney-Ernest G4 Peterson ABSTRACT: Method for the manufacture of a rocket motor having a glass-filament wound casing involving the steps of [54] METHOD FOR THE MANUFACTURE OF A forming the motor in at least two sections and providing for ROCKET MOTQR telescopic engagement of the casing of each section, providing 4 7 Drawmg abutting surface for the propellant charge, inhibiting the [52] US. Cl 86/1 abutting surfaces, telescopically engaging the casing of each [51] Int. Cl [$02k 9/04 section and bonding the inhibited and abutting surfaces of the [50] Field ofSearch 86/1; p pe ant Charge, and peripherally bonding the casing to 0 355 form a unitary motor.

METHOD FOR THE MANUFACTURE OF A ROCKET MOTOR This is a division of application Ser. No. 104.469. filed Apr. 20. i961.

This invention relates to gas-generating devices and more particularly to rocket motors and the like and their method of manufacture.

The present state of preferred art for the manufacture of solid propellant rockets consists in casting a propellant charge into a rocket chamber having certain characteristics and design to meet particular ballistic requirements. The desired thrust-time relationship is achieved through the geometrical configuration of the propellant charge. The rocket motor is designed for a specific end application and the performance, in most cases. is such that it cannot be appreciably altered for use in other applications. Furthermore. many propellant charges which have been designed have significant amounts of slivers which represent unusable propellant. in most instances where sliverless designs have been employedQthe charge configurations have been such that considerable amount of thermal insulation has been required to protect the rocket motor chamber from the high gas temperatures at. least in certain areas. Still further. in the manufacture of solid propellant charges having internal configurations. it is necessary to employ a mold to achieve the particular'grain geometry desired. A part of this mold involves a core which must pass through the opening in the rocket chamber. it is often necessary to have a core of significant diameter to achieve the charge configuration. ln contrast to this. the rocket designer always seeks to limit the diameter of the access to the motor in order to keep closure weights at a minimum. The large cores required and the desire for a small opening in the chamber are not compatible. Although this problem has been solved by the use of multisegment cores of complex geometry. such systems are. extremely expensive and require a considerable amount of assembly and disassembly time. Moreover. an additional problem which has been experienced in the past in the casting of propellant charges through restricted openings-has been the difficulty encountered in achieving proper casting powder loading density to produce satisfactory propellant.

From the foregoing. it is evident that there is considerable need for advances in the art for the manufacture of rocket motors and particularly where high'performance rocket motors are desired. Furthermore. it is evident that there is-considerable need to attain objectives that would render such high-performance rocket motors more versatile for application without resorting to techniques requiring a major redesign. It will be apparent from the detailed description given hereinafter andfrom the. appended claims that these needs are accomplished in accordance with this invention.

Generally described. in accordance with the present invention, multiple layers of propellant having different ballistic properties are arranged radially within the combustion chamber of a rocket motor. the highest burning rate system being in the minor diameter of the charge and the burning. rates decreasing in each succeeding layer with the lowest being in contact with the chamber wall. By sucha technique it has been possible to achieve rocket performance which is essentially of a neutral characteristic. i.e.. the thrust being substantially of a constant level during burningThe thrust-time curve obtained is of a sawtooth characteristic. That is. the thrust increases duringthe burning of a givenpropellant layer until its outer extremity is reached. At that point. a propellant havinga lower burning rate is uncovered and ignitedand the thrust level decreases to a new value. The configuration then becomes progressive during the burning of this new propellant layer and the sequence of ignition is repeated. Thepropellant layers extend longitudinally and intersect the forward and aft domes-of the rocket motor. This design yields essentially a simultaneous complete burnout of the last layer of the propellant charge over the interiorcylindrical surface of the motor wall, thereby achieving a major reduction in inert weight by the elimination of thermal insulation normally required to protect the combustionchamber in this area.

Moreover. the present invention includes the manufacture of the motor components in multiple segments and the bonding thereof. The segments are joined at areas along the cylindrical portion of the motor with a lightweight adhesive bond. This technique has made it possible to achieve motor manufacture with free access to the interior of the chamber during propellant processing. This allows for much better loading of casting powder into the mold and greatly simplifies the processing equipment required. Subsequent to the segment manufacture. the unit is prepared for bonding. The exact number of joints to be employed in a given application depends on the particular end use.

A preferred embodiment of the invention has been chosen for purposes of illustration and description and is shown in the accompanying drawings wherein reference symbols refer to like parts wherever they occur and wherein:

HO. 1 is a longitudinal. part sectional and part elevational view of agas-generating device in the form of a rocket motor illustrating the application of the invention;

FIG. 2 is a full sectional view taken along the section 22 of FIG. 1;

FlG. 3 is a graph depicting the pressure-time relationship of the multiple layer propellant illustrated and described in conjunction with FIGS. 1 and 2;

FIG. 4 is a fragmentary, longitudinal. sectional view depicting the formation of the casing for the rocket motor illustrated in FIGS. 1 and 2;

FlG. 5 is a longitudinal. sectional view depicting fluid pressure means for removing the forward section of the casing from the mandrel illustrated in FIG. 4;

FIG. 6 is a longitudinal, sectional view depicting fluid pressure means for removing the aft section of the casing from the mandrel illustrated in FIG. 4; and

H6. 7 .is an enlarged. longitudinal. sectional view of the bond joint utilized in bonding the forward and aft sections of the rocket motor casing and propellant together.

Referring now to the drawings. the rocket motor in ac cordance with the present invention comprises a lightweight oriented glass fiber reinforced casing 2. a molded asbestosfilled phenolic nozzle 4 with a graphite throat insert 6. and a high specific impulse propellant charge 8. The casing 2 was fabricated of helically wound fiberglass roving embedded in a matrix of epoxy resin and had an aluminum adapter at both the forward end and the aft end 10 and 12. respectively. The casing unit was cut circumferentially inthe cylindrical section. making a forward section and an aft section 14 and [6. respectively. The nozzle 4'was a standard convergent-divergent type nozzle made of an asbestos-filled phenolic material with the graphite insert 6 at the throat integral therewith. The exterior surface of the nozzle was wrapped with fiberglass roving embedded in a matrix of epoxy resin to achieve the necessary structural strength.

The propellant charge 8 consisted of five concentric layers of high-impulse modified double-base propellant. The propellant positions are shown in FIG. 2 and their compositions given in Table I. These layers were positioned to yield the pressure-time relationship shown in FIG. 3. From this graph it is notedthat the pressure peaks do not deviate appreciably from the mean value.

The fabrication of the rocket motor in accordance with this invention consists of winding and curing the casing 2 with the adapters l0 and 12 in place, inspection, installation of the forward and aft insulators l8 and 20. respectively, powder embedding. bonding the forward phenolic core support and inhibitor 22, bonding the aft inhibitor 23. and air testing, powder loading. casting, and curing. The propellant charge 8 is then machined'to final dimensions and radiographically inspected. Radial plane inhibitors or discs 24 and 26 are bonded in place and the forward and the aft sections 14 and 16, respectively, are bonded together. A suitable igniter represented by 28 and the nozzle 4 are then installed. This fabrication technique is more fully described as follows.

The winding of the casing 2 consisted of the orderly wrapping of the resin impregnated glass filaments on to a rotating mandrel 30 with the adapters l and 12 affixed thereto with the wrapping proceeding over the outer flanges of the adapters to securely bond them to the casing. These filaments were wrapped in both the circumferential and helical directions to balance the stresses and give the lightest possible casing or chamber thereby formed. The forward and aft sections 14 and 16, respectively, of the casing 2 were wrapped on the single mandrel which had an increase in the outside diameter of 0.120 inch at the middle section to give an aft section of slightly larger inside diameter (see FIG. 4). After the casing 2 had been wrapped and cured. it was cut circumferentially at each transition point between the 16.000-inch diameter and the 16.120-inch diameter to give two separate parts; the forward part or section 14 being small enough. due to the difference in diameters, to be slipped inside the aft part or section 16. The two parts were bonded together in a later operation to give one complete chamber (this step is discussed in detail later). These two sections were visually and dimensionally inspected. Upon completion of the inspection, the units were started through processing. insulators l8 and 20. respectively, were bonded to the fore and to the aft domes of sections 14 and 16, The first step in bonding of the insulators was to remove any mold release agent. The smooth surface on the inside of the sections forming the chamber was then roughened by sanding and buffing. The insulators were then coated with primer and allowed to cure for 16 hours at 100 F. Upon completion of the cure step. epoxy-amine-flexibilized adhesive was then applied to both the mating chamber surfaces and the insulator surfaces and the parts pressed together and cured for 8l6 hours at to F. A barrier coat of epoxy-polyamid resin was then brushed on the inner surface of each of thc insulators. A second coat of opoxy-polyamid resin was then applied to the interior of the insulator and to the interior of the chamber wall of each section, and casting powder was embedded in this material. The cmbedment layer was from about one to about two powder granules in depth.

The next step in fabrication of the motor was to bond the forward core support 22 with epoxypolyamid resin to the insulator 18 on the forward end while the unit was in the'casting fixtures (there is no support on the aft section of the chamber). Casting powder was then loaded. The powder loading was accomplished in two steps. The first was to load the lowest burning rate powder E in the forward dome only. Then aluminum separators which were subsequently removed with the exception of the inner core members were placed in the forward and aft end of each section to allow the different type casting powders A to E. inclusive, Table l, to be loaded into each specified section to form concentric layers. The casting powders had a granule size in the order of about 0.06 inch. which size afforded good packing efficiency. Upon completion of loading. a vacuum was applied to the motor sections to remove moisture from the powder granules. This vacuum was held for 16 hours, after which casting solvent of nitroglycerin, triacetin. and 2nitrodiphenylamine was introduced in conventional manner simultaneously to all five casting powders with a vacuum applied to the casting powder during the operation. After casting. the two sections ofthe unit were allowed to cure for 4 days at 140 F.

More particularly. with reference to FIG. 7. when the cure step was completed and the inner core members removed. the unit was ready for the final steps of fabrication which were machining. radiographic inspection and bonding of the two sections together. The individual sections were machined to prepare them for bonding. The machining of the forward section consisted of contouring the forward end of the charge, machining the face of the charge 8 in this section, and removing approximately 4 inches of chamber wall so that the aft edge of the casing 14 projected one-sixteenth inch beyond the face of the charge. From this aft edge forward for 2 inches formed the male part for the bond joint in the bond area 32. The machining of the aft section consisted of machining the face of the charge 8 in this section, and removing all but 2 inches of the casing 16 forward of the charge face. This projection now formed the female part for the lap joint in the bond area 32. Upon completion of the machining, the two sec tions ofthe unit were examined by radiographic analysis to detect cracks, separations, and porosity. With examination showing the two sections of the unit to be acceptable. rubber discs 24 and 26 were applied over each machined face. These discs were coated with primer and bonded to the exposed charge faces with a polyurethane adhesive and were allowed to cure for 8-16 hours. Upon completion of the curing the fore-and-aft sections of the unit were ready to be bonded together with the casing thereof ready for telescopic engagement and the charges ready for abutment to each other.

The bonding of the units consisted of the following steps in the order named: (l) degrease telescopic bonding surfaces of casings l4 and 16 with acetone; (2) remove glaze on surfaces with steel wool or fine emory paper; (3) prime surfaces with epoxy-polyamid resin and allow to dry; (4) remove glaze from prime coat on surfaces with steel wool and fine emory paper; (5) apply epoxy-polyamid resin to both sides of a piece of glass tape 34; allow the resin to work into the tape until it .becomes translucent; (6) insert the adhesive-laden tape into casing 16; (7) slide casing 14 into position within casing 16 to mate the rubber discs 24 and 26; this procedure additionally removes large entrapped air pockets from the glass tape; (8) after the two sections are together, apply epoxy-polyamid resin to the outside of both sections in the bond area; (9) repeat step (5) and wrap the similarly prepared glass tape 36 tightly around the bond joint area; (10) again apply epoxypolyamid resin over the tape 36 and bond surfaces; l 1 wrap teflon tape (adhesive side out) around the bond joint area to prevent the adhesive from flowing out; (12) cover the teflon tape with masking tape; (13) cure the unit thus prepared at F. for 24 hours; and (14) after curing, inject polyurethane resin under pressure between the rubber discs 24 and 26 from the interior thereof to insure a tight seal and bond therebetween. The above described procedure gives a bond joint as shown in FIG. 7 after removal of the teflon and masking tapes. The aft phenolic charge inhibitor 23 was then bonded to the aft portion of the propellant charge adjacent to the throat of the nozzle with epoxy-polyamid resin. The igniter 28 and the nozzle 4. which included the graphite throat insert 6. were next assembled into the unit to give the completed rocket motor as shown in FIG. 1.

TABLE I.PROPELLANT CHEMICAL COMPOSITIONS (Percent Resor- Layer NC N G A1 A1 TA NDPA HMX einol HMX =Cyc1otetramethylene; tetranitl'amine.

The rocket motor heretofore described had a diameter of approximately 16 inchesand had a total length exclusive of the nozzle of approximately 64.8 inches. The propellant weight achieved in the motor was 576.3 pounds using the following approximate layer thicknesses given in inches: A

0.687; B 0.829; C 0.962; D 0.982; E 1.540; and with the thickness adjacent the forward end approximately 4.160 inches. The motor mass ratio, i.e., propellant weight over gross motor weight. was 0.942. The delivered specific impulse of the propellants utilized was 250 lb.-sec./lb. at 1000 p.s.i. and sea level. a

From the foregoing. it is evident that this invention may be carried out by the use of various modifications and changes without departing from its spirit and scope. For example. a particularly efficacious system for stripping the casing sections 14 and 16 from the mandrel 30 after cutting the casing 2 into the two parts was found to reside in hydraulic stripping which is more readily seen with reference to FIGS. 4. 5 and'6. in this system, mandrel spindles 38 and 40. respectively. were provided for centering of the mandrel 30 for rotative movement during the winding of the filament material thereon. more particularly. in accordance with the method. resins and filament materials disclosed in U.S. Pat. No. 2,843,153 to R. E. Young. The forward spindle 38 was ported and formed an integral part of the mandrel 30. The forward adapter was placed over the spindle 38 and was held securely in place on the mandrel by athreaded lock nut 42. A threaded centering cap 44 to insure firm centering was then screwed against the lock nut 42. This completed. the forward section was ready for winding. In a somewhat similar manner.the aft spindle 40. which also was an integral part of the mandrel 30. had a concentric casing 46 which was ported. The aft adapter 12 was placed over the concentric casing 46 and was held securely in place on the mandrel 30 by a threaded lock nut 48. This completed, the. aft section was ready for winding. The casing material was then wound over the mandrel including the flanged portion of the forward and aft adapters. and after curing. the casing was circumferentially cut to make a fore-andaft section as heretofore described. The two parts of the casing were now ready for stripping from the mandrel.

Referring now more particularly to FIGS. 5 and 6. for stripping the forward section of the casing, the members 42 and 44 (FIG. 4) were removed. A front strippinghead 50 was screwed onto the forward adapter 10 and pressure fluid lOO p.s.i.) medium was passed into the stripping head. The fluid pressure thus delivered passed through a plurality of ports represented by 52 and due to further confinement was then exerted against the interior side of the adapter 10. causingit and its casing to move forward and release from the mandrel. after which the whole section was easily removed. Similarly. for stripping the aft section of the casing. the member 48 (FIG. 4) was removed. A rear stripping head 54 was screwed onto the aft adapter 12 and pressure fluid 100 p.s.i.) medium was passed into the stripping head. The fluid pressure thus delivered passed through a plurality of ports represented by 56.and.due to fiirther confinement was then exerted against the interior side of the adapter 12. causing it and its casing to move rearward and release from the mandrel after which the whole section was easily removed. it will be appreciated that this system of hydraulic stripping is quite superior to mechanical stripping in that the fluid pressure medium tends to be forced between the mandrel and the adapters. thus progressively increasing the area of uniformly applied thrust. This prevents any localized overstressing or damage to the units as sometimes encountered with mechanical stripping.

ltis further evident that although the preferred embodiment exemplified herein utilized a cast aluminized double-base solid propellant grain having five layers, other solid propellant systems and number of layers may be utilized. For example, systems may be utilized in which fuel binders such as polyurethane, polysulfide or copolymer polybutadiene rubbers or highly plasticized thermoplastics such as polyvinyl chloride, or

-m0nopropellant binders including poly petrin acrylate are used in conjunction with crystalline fuels such as aluminum and/or crystalline oxidants such as ammonium nitrate. The number of layers of propellant to be employed in any given system is dependent on the amount of tolerance permissible in respect to deviation between the pressure peaks of the various layers and the mean value for the entire propellant mass. Additionally. techniques can be employed. due to freedom of access for casting in accordance with this invention. whereby the various layers may be distinct in respect to propellant systems to achieve the best possible combinations of physical and chemical characteristics. However. in all instances the in dividual concentric layers will have decreased burning rates relative to each other in radial progression. Although silica loaded Buna N rubber is a preferred insulating material. other insulating material such as phenolic-asbestos. phenolic-glass. epoxy-zirconia. and various rubbers may be utilized. In this respect. it will be noted that the thickness of the insulating material conforms to the duration of exposure to the hot gases of combustion internally generated with the thickest portion being innermost and uniformly diminishing to a feather edge at the constant radius surface of the motor chamber. Still further. in addition to the particular resins and adhesives disclosed in conjunction with the preferred embodiment of the invention. other materials suitable for this purpose may be utilized and in lieu of the injection principle disclosed for bonding the abutting inhibited charge surfaces. adhesive may be applied thereto prior to telescopic engagement of the sections.

it will be apparent from the foregoing that the advantages of the invention are multifold. The use of a method of manufacturc whereby the various segments of the rocket motor are bonded together late in manufacture yields a useful tool for the variation of rocket size. For certain applications it is possible to make a selection of various propellant charge lengths shortly prior to anticipated use. ln these instances the particular overall length desired is achieved by decreasing or increasing the number of segments for a given design. The technique of this invention lends itself readily to the manufacture of large grain configurations. in very large units the logistics involved in the handling of large heavy items is improved by the segmented manufacturing technique of this invention. A further advantage accruing through the multiple segment design is the fact that the interior of the rocket motor is readily accessible to accommodate necessary production devices such as cores in grain manufacturing fixtures. In conventional designs the diameter of the nozzle port has been the controlling factor for the introduction and removal of such equipment from the rocket motor. The propellant chamber design of the present invention with its full opening allows the rocket designer wide latitude in manufacturing the grain. in assembling inert parts into the chamber, and in optimizing lightweight nozzle attachment devices. Still further. the elimination of the major proportion of thermal insulation in accordance with this invention by utilizing the multiple propellant layer concept yields a significant reduction in inert weight.

We claim:

1. in the method for the manufacture of a substantially cylindrical high mass ratio rocket motor having a glass-filament wound and bonded casing containing a solid propellant charge. the improvement which comprises forming the rocket motor in at least two sections and providing for telescopic engagement of the casing of each section. providing abutting surfaces for the propellant charge. inhibiting the abutting surfaces. telescopically engaging the casing of each section and bonding the inhibited and abutting surfaces of the propellant charge, and peripherally bonding the telescopic portion of the casing internally and externally to form a unitary motor.

2. In the method for the manufacture of a substantially cylindrical high mass ratio rocket motor having a glass-filament wound and bonded casing containing a solid propellant charge, the improvement which comprises forming the rocket motor in two sections comprising a fore section and an aft section and providing for telescopic engagement of the casing of the sections. providing abutting surfaces for the propellant charge, inhibiting the abutting surfaces of each section. telescopically engaging the fore section and the aft section and bonding the inhibited and abutting surfaces of the propellant charge, and peripherally bonding the telescopic portion of the casing internally and externally to form a unitary motor.

3. The method as in claim 2 in which the propellant charge is cast into each section in the form of a plurality of concentric 

1. In the method for the manufacture of a substantially cylindrical high mass ratio rocket motor having a glass-filament wound and bonded casing containing a solid propellant charge, the improvement which comprises forming the rocket motor in at least two sections and providing for telescopic engagement of the casing of each section, providing abutting surfaces for the propellant charge, inhibiting the abutting surfaces, telescopically engaging the casing of each section and bonding the inhibited and abutting surfaces of the propellant charge, and peripherally bonding the telescopic portion of the casing internally and externally to form a unitary motor.
 2. In the method for the manufacture of a substantially cylindrical high mass ratio rocket motor having a glass-filament wound and bonded casing containing a solid propellant charge, the improvement which comprises forming the rocket motor in two sections comprising a fore section and an aft section and providing for telescopic engagement of the casing of the sections, providing abutting surfaces for the propellant charge, inhibiting the abutting surfaces of each section, telescopically engaging the fore section and the aft section and bonding the inhibited and abutting surfaces of the propellant charge, and peripherally bonding the telescopic portion of the casing internally and externally to form a unitary motor.
 3. The method as in claim 2 in which the propellant charge is cast into each section in the form of a plurality of concentric layers of propellant having decreased burning rates in radial progression and with the outermost layer extending across the forward end of the fore section.
 4. The method as in claim 2 in which the telescopic portion of the casing is peripherally bonded and strengthened by applying a first bonded member between the telescopic portions of the casing and applying a second bonded member over the telescopic portions of the casing. 